Geologic Map of the Devore 7.5' quadrangle, San Bernardino County, California

This data set maps and describes the geology of the Devore 7.5' quadrangle, San
Bernardino County, California. Created using Environmental Systems Research Institute's
ARC/INFO software, the data base consists of the following items: (1) a map coverage
containing geologic contacts and units, (2) attribute tables for geologic units
(polygons), contacts (arcs), and site-specific data (points). In addition, the data
set includes the following graphic and text products: (1) A PostScript graphic
plot-file containing the geologic map, topography, cultural data, a Correlation of Map
Units (CMU) diagram, a Description of Map Units (DMU), an index map, a regional
geologic and structure map, and a key for point and line symbols; (2) PDF files of this
Readme (including the metadata file as an appendix), Description of Map Units (DMU),
and the graphic produced by the PostScript plot file.
The Devore quadrangle straddles part of the boundary between two major physiographic
provinces of California, the Transverse Ranges Province to the north and the Peninsular
Ranges Province to the south. The north half of the quadrangle includes the eastern San
Gabriel Mountains and a small part of the western San Bernardino Mountains, both within
the east-central part of the Transverse Ranges Province. South of the Cucamonga and San
Andreas Fault zones, the extensive alluviated area in the south half of the quadrangle
lies within the upper Santa Ana River Valley, and represents the northernmost part of
the Peninsular Ranges Province.
There are numerous active faults within the quadrangle, including right-lateral
strike-slip faults of the San Andreas Fault system, which dominate the younger
structural elements, and separate the San Gabriel from the San Bernardino Mountains.
The active San Jacinto Fault zone projects toward the quadrangle from the southeast, but
its location is poorly constrained not only within the quadrangle, but for at least
several kilometers to the southeast. As a result, the interrelation between it, the
Glen Helen Fault, and the probable easternmost part of the San Gabriel Fault is
intrepretive. Thrust faults of the Cucamonga Fault zone along the south margin of the
San Gabriel Mountains, represent the rejuvinated eastern end of a major old fault zone
that bounds the south side of the western and central Transverse Ranges (Morton and
Matti, 1993). Rejuvenation of this old fault zone, including the Cucamonga Fault zone,
is apparently in response to compression in the eastern San Gabriel Mountains resulting
from initiation of right-lateral slip on the San Jacinto Fault zone in the Peninsular
Ranges.The structural grain within the San Gabriel Mountains, as defined by basement
rocks, is generally east striking. Within the Devore quadrangle, these basement rocks
include a Paleozoic (?) schist, quartzite, and marble metasedimentary sequence, which
occurs as discontinuous lenses and septa within Cretaceous granitic rocks. Most of the
granitic rocks are of tonalitic composition, and much of them are mylonitic. South of
the granitic rocks is a complex assemblage of Proterozoic (?) metamorphic rocks, at
least part of which is metasedimentary. The assemblage was metamorphosed to upper
amphibolite and lower granulite grade, and subsequently remetamorphosed to a lower
metamorphic grade. It is also intensely deformed by mylonitization which is
characterized by an east striking, north dipping foliation, and by a pronounced
lineation that plunges shallowly east and west.
East of Lytle Creek and west of the San Andreas Fault zone, the predominant basement
lithology is Mesozoic Pelona Schist, which consists mostly of greenschist grade
metabasalt and metagraywacke. Intruding the Pelona Schist, between Lytle Creek and
Cajon Canyon, is the granodiorite of Telegraph Peak of Oligocene age (May and Walker,
1989). East of the San Andreas Fault in the San Bernardino Mountains, basement rocks
consist of amphibolite grade gneiss and schist intermixed with concordant and
discordant tonalitic rock and pegmatite. Tertiary conglomerate and sandstone occur in
the Cucamonga Fault zone and in a zone 200 to 700 m wide between strands of the San
Andreas Fault zone and localized thrust faults northeast of the San Andreas. Most of
the conglomerate and sandstone within the Cucamonga Fault zone is overturned forming
the north limb of an overturned syncline. Clasts in the conglomerate are not derived
from any of the basement rocks in the eastern San Gabriel Mountains. Clasts in the
conglomerate and sandstone northeast of the San Andreas Fault zone do not appear to be
locally derived either. The south half of the quadrangle is dominated by the large
symmetrical alluvial-fan emanating from the canyon of Lytle Creek, and by the complex
braided stream sediments of Lytle Creek and Cajon Wash.
The San Andreas Fault is restricted to a relatively narrow zone marked by a pronounced
scarp that is especially well exposed near the east margin of the quadrangle. Two
poorly exposed, closely spaced, north-dipping thrust faults northeast of the San
Andreas Fault have dips that appear to range from 55ƒ to near horizontal. The
shallower dips probably are the result of rotation of initially steeper fault surfaces
by downhill surface creep. Between the San Andreas and Glen Helen Fault zones, there
are several faults that have north facing scarps, the largest of which are the east
striking Peters Fault and the northwest striking Tokay Hill Fault. The Tokay Hill
Fault is at least in part a reverse fault. Scarps along both faults are youthful
appearing.
The Glen Helen Fault zone along the west side of Cajon Creek, is well defined by a
pronounced scarp from the area north of Interstate 15, south through Glen Helen
Regional Park; an elongate sag pond is located within the park.
The large fault zone along Meyers Canyon, between Penstock and Lower Lytle Ridges, is
probably the eastward extension of the San Gabriel Fault zone that is deformed into a
northwest orientation due to compression in the eastern San Gabriel Mountains (Morton
and Matti, 1993). At the south end of Sycamore Flat, this fault zone consists of
three discreet faults distributed over a width of 300 m. About 2.5 km northwest of
Sycamore Flats, it consists of a 300 m wide shear zone. At the north end of Penstock
Ridge, the fault zone has bifurcated into four strands, which at the northwest corner
of the quadrangle are distributed over a width of about one kilometer. From the
northern part of Sycamore Flat, for a distance of nearly 5 km northwestward, a
northeast dipping reverse fault is located along the east side of the probable San
Gabriel Fault zone. This youthful reverse fault has locally placed the Oligocene
granodiorite of Telegraph Peak over detritus derived from the granodiorite.
The Lytle Creek Fault, which is commonly considered the western splay of the San
Jacinto Fault zone, is located on the west side of Lytle Creek. Lateral displacement
on the Lytle Creek Fault has offset parts of the old Lytle Creek channel; this offset
gravel-filled channel is best seen at Texas Hill, near the mouth of Lytle Creek, where
the gravel was hydraulic mined for gold in the 1890s.
The Cucamonga Fault zone consists of a one kilometer wide zone of northward dip-
ping thrust faults. Most splays of this fault zone dip north 25ƒ to 35ƒ.
The geologic map database contains original U.S. Geological Survey data generated by
detailed field observation and by interpretation of aerial photographs. This digital
Open-File map supercedes an older analog Open-File map of the quadrangle, and includes
extensive new data on the Quaternary deposits, and revises some fault and bedrock
distribution within the San Gabriel Mountains. The digital map was compiled on a
base-stable cronoflex copy of the Devore 7.5í topographic base and then scribed. This
scribe guide was used to make a 0.007 mil blackline clear-film, from which lines and
point were hand digitized. Lines, points, and polygons were subsequently edited at the
USGS using standard ARC/INFO commands. Digitizing and editing artifacts significant
enough to display at a scale of 1:24,000 were corrected. Within the database, geologic
contacts are represented as lines (arcs), geologic units as polygons, and site-specific
data as points. Polygon, arc, and point attribute tables (.pat, .aat, and .pat,
respectively) uniquely identify each geologic datum.